London: US President Barack Obama yesterday called on Yemen's President Ali Abdullah Saleh to "immediately" honour his promise to transfer power, amid rising violence between the government and the opposition.

"We call upon President Saleh to move immediately on his commitment to transfer power," Obama said, in a joint press conference with British Prime Minister David Cameron in London.

On Libya, Obama and Cameron promised a relentless and punishing pummelling of Muammar Gaddafi's forces. Obama ruled out a deadline for ending Nato's military assault and said it would be over "in a timely fashion".

"Ultimately this is going to be a slow, steady process in which we're able to wear down the regime forces," Obama said.

"I believe that we have built enough momentum that, as long as we sustain the course, we're on, he will step down."

Cameron rallied behind Obama saying what was needed in Libya was "patience and persistence".

The statements of mutual support came despite complaints among some Nato countries about the reduced US role since Nato took the lead after the initial days of the two-month-old campaign against Gaddafi.

"We've been extraordinarily successful in avoiding civilian casualties," Obama said at one point in the question and answer session.

He once again ruled out ground forces in Libya. "That means that sometimes we may have to be more patient than people would like," he conceded.

On the Middle East peace process, Cameron strongly supported Obama's recent speech in which the president explicitly endorsed a return to Israel's pre-1967 borders, along with mutually agreed-to land swaps, as the starting point for peace negotiations with the Palestinians.